1. Field of the Invention
The present invention relates to a wire electrical discharge machining apparatus and, more particularly, to a mechanism for preventing a wire electrode wound around a wire bobbin from loosening.
2. Description of the Prior Art
FIG. 10 shows an example of the structure of a conventional wire electrical discharge machining apparatus. In FIG. 10, the wire electrical discharge machining apparatus body 1 is provided with a wire bobbin assembly 3 composed of a wire bobbin with a wire electrode 2 wound therearound.
FIG. 11 shows the details of the wire bobbin assembly 3. In FIG. 11, the wire bobbin 4 with the wire electrode 2 wound therearound is provided with a loosening preventive means 5 for preventing the wire electrode 2 wound around the wire bobbin 4 from loosening and bulging out of the outer periphery of the wire bobbin 4. A retaining means 6 for retaining the loosening preventive means 5 in a state of being apart from the wire bobbin 4 is secured to the wall surface of the wire bobbin assembly 3. The retaining means 6 is composed of, for example, a magnet. In the feeding route of the wire electrode 2, a wire tension reel 7 is provided which rotates in the direction of rewinding the wire electrode 2 while the torque thereof is controlled by an AC motor or the like. The returning force of the wire tension reel 7 provides the wire electrode 2 with a desired tension.
FIG. 12 shows the details of the loosening preventive means 5. In FIG. 12, the loosening preventive means 5 includes a wire presser plate 8 with the side surface thereof in contact with the outer peripheral surface of the wire electrode 2 which is wound around the wire bobbin 4, an arm 9 for supporting the wire presser plate 8, a hinge 10 with the one end thereof attached to the arm 9 and having a spring (not shown) for pressing the arm 9 to the side of the wire bobbin 4, and a fixture 11 for fixing the other end of the hinge 10 to the wire electric discharge machining apparatus body 1.
In FIGS. 11 and 12, the same numerals are provided for the elements which are the same as those shown in FIG. 10, and explanation thereof is omitted.
The operation of the conventional apparatus will now be explained. At the time of initial setting of the wire electrode 2 for starting electrical discharge machining, the loosening preventive means 5 is manually removed from the wall surface by the operator against the retaining force of the magnet of the retaining means 6. The loosening preventive means 5 presses the presser plate 8 against the outer winding peripheral surface of the wire electrode 2 around the wire bobbin 4, thereby stretching the wire electrode 2 in the feeding route while preventing the wire electrode 2 from loosening and bulging out of the wire bobbin 4. In this way, the initial setting is completed.
Electrical discharge machining is then started. If the presser plate 8 is in the state of being pressed against the outer winding peripheral surface of the wire electrode 2, the pressing force acts on the rotation of the wire bobbin 4 as a load so as to fluctuate the tension of the wire electrode 2, thereby making precise machining impossible. To prevent this, when electrical discharge machining is started, the loosening preventive means 5 is again separated from the wire bobbin 4 so as to be retained by the retaining means 6 and to eliminate the pressing force.
In a conventional wire electrical discharge machining apparatus having the above-described structure, it is necessary to keep the loosening preventing means 5 apart from the outer winding peripheral surface of the wire electrode 2 during electrical discharge machining. If the wire electrode 2 stops feeding during this period for some cause, the wire electrode 2 loosens and bulges out of the wire bobbin 4, thereby greatly impairing the operability. In this case, the conventional countermeasure is to prevent the loosening of the wire electrode by stopping the rotation of the wire tension reel 7 so as to prevent the wire electrode 2 from loosening due to the reverse rotation (rotation in the direction of rewinding). In order to stop the rotation of the wire tension reel 7, however, it is necessary to provide the reel 7 with a large braking force by, for example, applying a voltage in the reverse direction to the motor so as to obtain the maximum stopping torque and increasing the frictional resistance. For this reason, if the force applied to the reel 7 is small, the rotation of the reel 7 is not stopped completely due to the inertia, thereby loosening the wire electrode 2. On the other hand, if the force is too strong, the wire electrode 2 sometimes leaps up or the wire springs out of the reel 7 due to the reaction. Even after the reel 7 stops rotating, the wire bobbin 4 races due to the inertia or reversely rotates, thereby disturbing the winding of the wire electrode 2.